1. Field of the Invention
The present invention relates generally to Coriolis type mass flowmeters and more particularly to an improved single tube flowmeter which is substantially insensitive to external pipeline vibrations, and is substantially insensitive to external pipeline stresses and distortions. The flowmeter includes an intermediate section of conduit which functions to prevent mechanical coupling of the Coriolis response to the external piping and provides dynamic counterbalancing of internally generated Coriolis forces, The flowmeter does not require the use of a support structure.
2. Brief Description of the Prior Art
There is a continuing need for more accurate and efficient meters for determining the mass flow rate of fluids and/or liquid-solid slurries flowing through a pipeline or conduit. Coriolis-effect type mass flow rate meters used for this purpose are well known in the prior art. For example, U.S. Pat. No. 4,127,028 to Cox et al. discloses a mass flowmeter comprised of a pair of generally U-shaped tubes that are spaced apart in parallel with the inlet and outlet ends of each U-tube fixedly cantilever-mounted and with the corresponding bight ends free to move relative to one another. The tube's bight ends are connected by a drive mechanism which oppositely reciprocates the tubes like the tines of a tuning fork. A sensor is placed at the corresponding side leg of each U-tube.
Micro Motion.RTM. Model DL Mass Flowmeter Information and Specification Manual discloses a Coriolis type mass flowmeter similar to the '028 Cox device wherein a pair of generally U-shaped tubes are spaced apart with the inlet to the upstream U-tube and the outlet from the downstream U-tube fixedly mounted and with the corresponding bight ends free to move relative to one another. The Micro Motion.RTM. device and the '028 Cox device differ in that in the Cox device the connecting linkage between the upstream U-tube and the downstream U-tube are fixedly mounted to the device base fixture. In the Micro Motion.RTM. device, the tubes are formed from what appears to be a continuous length of conduit, the ends of which are fixedly mounted to the support structure.
U.S. Patent No. Re. 31,450 to Smith discloses a Coriolis type mass flowmeter similar to the Micro Motion.RTM. and Cox devices but is comprised only of a single U-shaped tube fixedly attached to a support in a cantilever fashion with the bight end free to move. At Smith '450, col. 3 lines 44-51, the function of the cantilevered beam-like mounting of the U-shaped tube is stated:
"The cantilevered mounting of the "U" shaped conduit is of more than passing significance. In the instance in which distortion is measured, such mounting provides for the distortion resulting from the Coriolis forces to be offset substantially entirely by resilient deformation forces within the conduit free of mechanical pivot means other than flexing of the conduit."
U.S. Pat. No. 4,957,005 to Yard discloses a Coriolis type mass flowmeter which is comprised of a series of loops formed in a single length of tubing with no joints. The loops, arranged in order, include an inlet isolation loop, an upstream metering loop, a downstream metering loop, and an outlet isolation loop. The junction between the inlet isolation loop and the upstream metering loop, the junction between the two metering loops, and the junction between the downstream metering loop and the outlet isolation loop are all rigidly mounted to a single bracket. The bracket is free to move relative to a support structure for the meter, thereby isolating the double loop structure from external forces and vibrations.
U.S. Pat. No. 5,020,375 to Back-Pedersen et al. discloses a Coriolis type mass flowmeter comprised of an upstream measuring tube loop, a downstream measuring tube loop, an upstream or inlet final tube, a downstream or outlet final tube, a first block, a second block, an inlet connecting tube, and an outlet connecting tube. The two measuring loops are formed in a single length of tubing bent several times. The transition point between the two measuring loops, the inlet to the upstream measuring loop, and the outlet from the downstream measuring loop are all captured in the second block. An end of the inlet connecting tube is connected to the inlet to the upstream measuring loop, and this connection is captured in the second block. In a similar manner, an end of the outlet connecting tube is connected to the outlet from the downstream measuring loop and this connection is also captured in the second block. The opposite ends of the inlet and outlet connecting tubes are captured in the first block as are one end of each of the upstream and downstream final tubes. The connecting tubes are attached to their corresponding final tube sections.
U.S. Pat. Nos. 4,660,421, 4,711,132, and 4,984,472 to Dahlin et al. disclose a Coriolis type mass flowmeter that is comprised of a single tube length formed into at least one helically shaped loop, wherein the inlet and outlet ends of the tube are fixedly mounted to a support and disposed at opposing ends of the flowmeter.
A significant disadvantage with prior art flowmeter devices such as those mentioned above is that the support or base of the meters must be mounted in a rigid and secure manner. The prior art devices are thus very sensitive to pipeline stresses, pipeline distortions and to the mounting configuration of the meter and will give inaccurate readings if the device is improperly mounted or supported. In other words, the prior art devices have a high mounting sensitivity. For example, the Micro Motion.RTM. Model DL meter is very sensitive to the mechanical coupling of the meter to the outside world. (Refer to Micro Motion.RTM. Model DL Information and Specification Manual, Section 2.4.) If the mounting is changed, the response motion and the excitation motion can couple to the external environment resulting in flow rate errors caused by zero-flow offset shifts and calibration shifts. This coupling sensitivity of the meter to the external environment is evident in the Cox, Dahlin, and Yard devices. Although attempts have been made in the design of these prior art devices to enhance the isolation capability of their meters by using support structures that are heavy and/or rigid, such structures materially add to the cost of the device and complicate the installation.
Another disadvantage of the prior art metering devices is that stresses in the flowmeter support or base are readily transmitted to the metering tubes resulting in inaccurate mass flow rate readings. For example, the Cox type of prior art mass flow meter is highly sensitive to external vibrations which may interfere with accurate measurement. In order to isolate the meters from the outside environment, attempts are made to make the tubing support structure as rigid as possible. This also results in massive and/or expensive supports that are still sensitive to mounting and piping support configurations and accommodations.
Also, some prior art devices, specifically the Yard and the Back-Pedersen et al. devices, attempt to isolate the external vibrational environment by physically decoupling the metering tube section from the mounting section. In the Yard device, as described earlier, a continuous double loop metering section is mounted at these points to a rigid bar that "floats" with respect to the support structure of the metering device itself. In the Back-Pederson et al. device, a continuous tube, double loop metering section is tied into a block in hydraulic communication with another block which is used to attach the metering device to the external system piping or tubing. In both devices the metering section of the flowmeter is isolated, to some extent, from the external vibrational environment. Also, in both devices, the inlet and the outlet from the metering tube loops, and the junction between the upstream and downstream metering loops are all tied together in one rigid block structure. A problem with these configurations is that they do not cancel, or dynamically counterbalance, the forces that result during excitation and response of the sensing tubes thereby resulting in inaccurate flow rate readings.
Yet another disadvantage of the '028 Cox and the Micro Motion.RTM. devices is that the oscillatory drive motion creates significant bending stresses at the attachment points of the U-tubes, leading to a danger of failure due to stress-induced cracking and corrosion.